1. Field of the Invention
The present invention relates to a toner, a two-component developer, a developing device and an image forming apparatus.
2. Description of the Related Art
In recent years, efforts have been made extensively in an attempt to improve the quality of images formed by visualizing the latent images by using an image forming apparatus. As one of the concrete tendencies, the developer has been improved, particularly, by decreasing particle sizes of the toner in order to enhance the resolution and vividness. However, a decrease in the particle size of the toner is accompanied by a decrease in the transfer property and cleanness often deteriorating the quality of images.
In order to cope with, the above problems, it has been attempted to improve the transfer property and cleanness by adding an external additive as spacer onto the surfaces of the toner. However, the function of the spacer cannot be drawn to a sufficient degree unless the particle size and properties of the external additive are controlled.
If the average primary particle size of the external additive is too small the spacer effect is not obtained between the toner and the surface of the photoreceptor drum or the transfer belt (inclusive of both the transfer system directly onto the paper or the intermediate transfer belt system). Therefore, the adhering force of the toner increases and the cleanness cannot be ensured. If the average primary particle size is too large, the number of the external additive particles of large particle sizes increases and the amount of specific charge of the toner decreases. This is attributed to that the spacer effect becomes too great between the toner and the carrier due to the external additive, whereby the contact becomes defective between the toner and the carrier and the electric charge becomes poor. It is further considered that the external additive separates away from the toner particles in increased amounts making it difficult to ensure the amount of specific charge of the toner.
Japanese Unexamined Patent Publication JP-A 2004-102236 discloses a technology for obtaining the inherent function of the external additive by using, as an external additive for toner, fine oxide particles of substantially a spherical shape containing, at least, silicon element, having a number average primary particle size R of 30 to 300 nm, a standard deviation σ in the particle size distribution R of R/4≦σ≦R, having a circularity degree SF1 of 100 to 130, a circularity degree SF2 of 100 to 125, the fine oxide particles exhibiting a spacer effect to a sufficient degree, preventing the additive from being buried at the time when the toner is preserved at high temperatures or when the toner is deteriorated by vigorous stirring, while suitably setting the ratio of containing fine oxide particles of large particle sizes, intermediate particle sizes and small particle sizes, ensuring fluidity relying upon the particles of small particle sizes, effectively drawing out the spacer effect relying upon the particles of intermediate particle, large particle sizes, improving the fluidity of the toner, enhancing the affinity of the toner and of the fine oxide particles, and preventing the separation of the fine oxide particles from the toner.
However, JP-A 2004-102236 is not giving consideration to the amount of water in the fine oxide particles. The fine oxide particles having the average primary particle size of about 100 nm usually contain water in an amount of 5 to 8%. If the amount of water increases in the fine oxide particles, the electric charge leaks to the surfaces of the carrier via the fine oxide particles causing the occurrence of such problems as a decrease in the amount of electric charge of the toner through the endurance printing test, deterioration in the image quality and scattering of toner is the apparatus. Further, the fine oxide particles have a wide particle size distribution. If the amount of addition of the fine oxide particles is increased in order to ensure cleanness, therefore, the amount of fine oxide particles of small particle sizes increases, adversely affecting the fixing property.